The present invention relates generally to a solid state laser apparatus, and more particularly to a solid state laser apparatus which emits a laser beam from an optically-pumped solid laser medium.
Applications of solid state lasers encompass various fields, such as laser machining applications, measurement, medical science, etc., because of their small size and easiness of handling and operation. However, several problems still exist with the current solid state lasers. For example, a critical problem is reducing the effects caused by heat generated in the solid laser medium. Since the solid state laser apparatus usually cools its rod-shaped laser medium from the circumference of the laser rod, the laser rod has a high temperature in the center of the laser rod and a low temperature in the periphery. In the most popular YAG laser, this temperature gradient causes a convex lens effect where the refractive index is high in the center of the YAG rod, since the temperature variation rate of refractive index dn/dT of YAG is positive. Adding the effect of thermal expansion in the center of rod end face, the laser medium itself shows the convex lens effect, which is called "thermal lens effect."
The thermal lens effect prevents convergence of laser beam, and hinders the creation of a high quality laser beam. Thus, it is necessary to reduce the thermal lens effect as much as possible in order to produce a high quality laser beam.
A well known method for solving this problem involves using a slab solid state laser medium. A first pair of oppositely-facing side faces of the solid state laser slab are optically polished. The laser beam is then propagated in a zig-zag path through the laser slab repeating total reflection on the optically polished side faces. This zig-zag light path cancels the thermal lens effect in the direction perpendicular to the reflection plane. Additionally, the thermal lens effect in the direction perpendicular to a second pair of oppositely-facing side faces may be avoided by tightly covering the second pair of faces with heat insulators so as to minimize the temperature gradient in the direction perpendicular to the second pair of faces.
The above-described solution is effective in obtaining a high quality laser beam in the zig-zag propagation direction. However, it is difficult to improve beam quality in the direction parallel to the total reflection planes by using the slab laser medium, since it is impossible in practice to perfectly insulate the side faces of the slab laser medium thermally, and since optical properties in the direction parallel to the total reflection planes depend on thermal stress and thermal deformation of the total reflection planes.
Another known method for suppressing the thermal lens effect uses laser diodes (LDs) for pumping the laser media in place of excitation lamps which have been used widely. Since the LDs emit light only at a frequency which is effective for laser pumping, the laser beam from the LDs generates less heat in the laser media and effectively suppresses the thermal lens effect. However, the heat generated by the LDs is reduced, at the most, to only about one third of the heat generated by the excitation lamps, and further improvement is unexpected.
Yet another prior art method utilizes laser crystals with a negative dn/dT for suppressing the thermal lens effect. A negative dn/dT causes a concave lens effect which cancels the convex lens effect caused by swelling of the end faces of the solid laser medium. Crystals with a negative dn/dT include, e.g., LiYF.sub.4 doped with Nd.sup.3+ (cf. J. E. Murray: IEEE J. Quantum Electron., Vol. QE-19 (1983) pp. 488-491, H. Vanherzeele: Optic Letters, Vol. 13 (1988) pp. 369-371, G. Cerullo, et al.: Optics Communications, Vol. 93 (1992) pp. 77-81, etc.), Er:YLF (cf. R. C. Stoneman, et al.: IEEE J. Quantum Electron., Vol. 28 (1992) pp. 1041-1045), Tm:Ho:YLF (cf. B. T. Mcguckin, et al.: IEEE J. Quantum Electron., Vol. 28 (1992) pp. 1025-1028), Cr:LiSAF (LiSrAlF.sub.6), and Cr:LiCAF (LiCaAlF.sub.6) (Cr:LiSAF and Cr:LiCAF: cf M. D. Perry, et al.: Laser Focus World, September (1993) pp. 85-92). However, even though the thermal lens effect in a particular direction in a plane perpendicular to the propagation direction of the laser beam may be eliminated through the use of crystals with a negative dn/dT, the thermal lens effect in other directions fin the perpendicular plane remains uncancelled, since the optical properties of the crystals vary depending on the particular direction in the plane perpendicular to the laser beam propagation direction.